In the field of radio communication in cellular networks, the term “User Equipment, UE” is commonly used and will be used in this disclosure to represent any user-controlled wireless terminal, mobile phone, tablet or device capable of radio communication including receiving downlink signals transmitted from a radio node and sending uplink signals to the radio node. Further, the term “radio node”, also commonly referred to as a base station, e-nodeB, eNB, etc., represents any node of a wireless cellular network that can communicate uplink and downlink radio signals with UEs. The radio nodes described here may, without limitation, include so-called macro nodes and low power nodes such as micro, pico, femto, Wifi and relay nodes, to mention some customary examples. Throughout this disclosure, the term “BS” or “eNB” is often used to denote a radio node.
In a cellular network for radio communication, a Time Division Duplex (TDD) configuration of subframes may be used for uplink and downlink transmissions in cells where consecutive subframes are comprised in a repeatable radio frame. The subframes are reserved for uplink transmissions from User Equipments (UEs) to a serving radio node and for downlink transmissions from the radio node to the UEs such that uplink and downlink transmissions do not occur at the same time within the cell. A subframe is basically defined by a preset time period of a certain length, typically 1 millisecond (ms), and each subframe may comprise two time slots of 0.5 ms each. Further, a radio frame comprises a predefined number of consecutive subframes, e.g. ten subframes. In such a network, different radio nodes are able to use different configurations of subframes, e.g. depending on the current need for uplink and downlink radio resources, respectively.
An example of different TDD configurations that can be used by radio nodes in different cells is shown in the table of FIG. 1 comprising seven different TDD configurations 0-6 each having ten subframes 0-9 including downlink subframes “D”, uplink subframes “U” and so-called special subframes “S”. The special subframes S are configured with one part reserved for downlink, another part reserved for uplink, and a guard period between the two parts allowing neither uplink nor downlink. It can be seen in this example that the first three subframes 0-2 and subframe 5 are reserved for special S, uplink U, and downlink D, respectively, in all TDD configurations 0-6, while the remaining subframes 3, 4, 6-9 can vary in different TDD configurations. The latter subframes 3, 4, 6-9 may be referred to as flexible subframes having a variable link direction, and the former subframes 0-2 and 5 may be referred to as static or fixed subframes having a fixed link direction.
In this disclosure, the term “flexible subframe” thus denotes a subframe in which the direction of transmission, i.e. downlink or uplink, may differ between different cells so that the flexible subframe may be used for downlink transmission in one cell and for uplink transmission in another cell. Further, a flexible subframe may differ from one radio frame to another in the same cell so that the flexible subframe is used in the cell for downlink in one radio frame and for uplink in another radio frame. Thereby, transmissions in flexible subframes may, at least in some radio frames, cause interference between different neighboring cells as follows. In this disclosure, the expression “during a flexible subframe” should be understood as in the flexible subframe or in a subframe that overlaps in time with the flexible subframe depending on if the UE is served by the radio node that applies the subframe scheme with the flexible subframe or if the UE is served by a neighboring radio node that applies a subframe scheme with a subframe that overlaps or coincides in time with the flexible subframe.
When different TDD configurations are used in two neighboring cells, interference may occur across the cells during a flexible subframe where downlink is permitted in one cell and uplink is permitted in the other cell at the same time. In this description, the term “neighboring cells” means that they are close enough to one another so that transmissions in one cell can potentially cause interference in the other cell. Interference due to different TDD configurations in neighboring cells can be either 1) downlink to uplink interference when a downlink transmission from a radio node of one cell disturbs an uplink reception in a radio node of the other cell during a flexible subframe, or 2) uplink to downlink interference when an uplink transmission from a UE in one cell disturbs a downlink reception in a UE in the other cell during a flexible subframe. Of these two scenarios, 1) refers to interference between radio nodes which is more or less predictable and this interference can be controlled quite accurately since the radio nodes in the neighboring cells are in fixed positions relatively far away from each other such that the downlink signals from one radio node are not very strong when received in the other radio node.
On the other hand, scenario 2) above refers to interference between UEs which is more unpredictable since the UEs move around and may sometimes be located quite near each other while being served by different radio nodes, e.g. when both are located close to the borders of their respective cells. This scenario is illustrated in FIG. 2 where a first UE denoted “UE1” is located near the border of a first cell 1 served by a first radio node “BS1” using a TDD configuration allowing the UE1 to transmit uplink signals “UL1” in a certain subframe. At the same time, a second UE “UE2” is located near the border of a second cell 2 served by a second radio node “BS2” using another TDD configuration allowing UE2 to receive downlink signals “DL2” in the same subframe, thus being a flexible subframe in this context. Since UE1 and UE2 happen to be quite close to one another but relatively far away from their respective radio nodes, the uplink signals UL1 transmitted with high power from UE1 will interfere strongly with the relatively weak downlink signals DL2 received by UE2 during the flexible subframe. This UE to UE interference “I” is illustrated by a dashed arrow.
FIG. 3A shows two examples of TDD configurations which can cause UE to UE interference across neighboring cells 1 and 2. In cell 1, TDD configuration 1 of FIG. 1 is used and in cell 2, TDD configuration 2 of FIG. 1 is used. It can be seen in both of FIG. 1 and FIG. 3A that flexible subframes 3 and 8 are configured differently in the two cells such that they are uplink subframes in cell 1 and downlink subframes in cell 2, hence potentially causing UE to UE interference I from cell 1 to cell 2, illustrated by dashed arrows in FIG. 3A. In this case, UE1 can be called an “aggressor UE” and UE2 can be called a “victim UE”. Likewise, cells 1 and 2 can be called “aggressor cell” and “victim cell”, respectively. It is thus a problem that, in a radio communication network allowing different TDD configurations with one or more flexible subframes in different cells, downlink radio signals received by a victim UE in a victim cell during a flexible subframe, may be subjected to interference caused by an uplink transmission from an aggressor UE in an aggressor cell during that subframe, e.g. depending on the relative distance and locations of the UEs which are typically unpredictable.
In wireless communications, Channel State Information, CSI, refers to channel properties of a radio communication link. This information basically describes how a signal propagates from the transmitter to the receiver. The CSI makes it possible to adapt transmissions to current channel conditions, which may be helpful for achieving reliable communication with high data rates in a cellular network. The CSI needs to be estimated at the receiver, typically the UE, and it is usually quantized and reported back to the transmitter, typically the radio node serving the UE. This report is commonly referred to as CSI feedback. A CSI report may comprise a Channel Quality Indicator, CQI, a Precoding Matrix Indicator, PMI and/or a Rank Indicator, RI.
CSI feedback is typically used to support the performance of a wireless access network in various respects. For example, it is commonly used as a basis for different RRM functionalities such as scheduling, link adaptation as well as interference coordination. It can also be used for rank and precoding matrix recommendations for MIMO transmission. In LTE, two CSI reporting schemes are supported: periodic CSI reporting and aperiodic CSI reporting. The following disclosure relates to aperiodic CSI reporting for a TDD system where fast or flexible UL/DL reconfigurations are employed.
It is expected that wireless data traffic will become more and more localized in the future, where most UE users will be in hotspots, or in indoor areas, or in residential areas. These UE users will thus typically be located in clusters within a limited area of a cell served by a radio node, and the UEs will produce different UL and DL traffic at different times to and from the radio node, respectively. This essentially means that a dynamic feature to adjust the UL and DL resources to instantaneous (or short term) traffic variations would be required in future local area cells. In this case, a TDD system which has the flexibility to dynamically allocate the UL/DL resources depending on current traffic situation becomes very attractive.
As described above, today, there are seven different TDD UL/DL configurations defined in LTE, shown in the table of FIG. 1, providing a range of 40%-90% resources for DL. It can be seen in FIG. 1 that for example TDD configuration 5 has much more resources for DL than, say, TDD configuration 0. In current specification, the UL/DL configuration is semi-statically configured, thus it may not well match the instantaneous traffic situation which may vary quite rapidly. Faster TDD reconfigurations, hereafter referred to as “dynamic TDD”, have shown good performance potentials in both UL and DL, especially at low to medium system load, and dynamic TDD will become a standardized feature of LTE Rel-12. It should be noted that more TDD configurations than the ones listed in the table in FIG. 1 may be introduced in the future. The herein suggested solution is not limited to the existing TDD configurations; rather it is equally applicable to new configurations defined in future.
Different signaling methods are being considered to support dynamic TDD reconfigurations with different time scale. Theoretically, each subframe could be allocated as either UL or DL. However, this would pose big challenges to operations like DL/UL switching, random access, radio link monitoring, handover, etc. Moreover, it would be virtually impossible to achieve backward compatibility with legacy UEs. Therefore, it is more practical to dynamically change between UL and DL among a subset of the subframes, e.g. by changing between the different TDD configurations in FIG. 1. In this case, the subframes can be divided into two types: static subframes and flexible subframes. The static subframes have fixed link directions for all TDD configurations, while flexible subframes can be either UL or DL in different TDD configurations, and can thereby be dynamically changed between UL and DL, e.g. by change of TDD configuration for a cell, as described above.
It may be up to the eNB to configure the set of flexible subframes depending on the traffic situation. One possible way is to signal two different UL/DL TDD configurations to a UE, such that the flexible subframes are determined implicitly by the two reference TDD configurations i.e., as said above, the subframes in which the link directions in the two TDD configurations may be different are defined as flexible subframes. FIG. 3B shows an example where reference TDD configuration 0 is used for UL and reference TDD configuration 2 is used for DL. In this example, the static downlink subframes, which may include normal subframes and special subframes, are subframes 0, 1, 5 and 6, while the static uplink subframes are subframe 2 and 7, the subframes 0-2 and 5-7 thus being static subframes with fixed link directions. The remaining subframes 3, 4, 8 and 9 are flexible subframes which can be used for either uplink or downlink transmissions.
Considering UE reception in the two types of subframes, the interference situations may be different in different subframes, e.g. as described above with reference to FIGS. 1-3B. In static DL subframes, the inter-cell interference is generated by neighboring eNB(s), while in flexible subframes the inter-cell interference could either be generated by neighboring eNB(s) or by certain UE(s) served by the neighboring eNB(s) which are currently scheduled for UL transmissions. To deal with the above different interference situations, separate CSI measurements should be employed for the two types of subframes so that DL scheduling as well as link adaptation can be properly performed for both types of subframes on the basis of the respective CSI measurements. Thus, there is a need for mechanisms for handling such separate CSI measurements.